为什么这段代码会抛出 NullPointerException? [英] why does this code throw a NullPointerException?

问题描述

最终我得到了答案，但它让我困惑了一段时间.

Eventually I got the answer, but it puzzled me for a while.

为什么下面的代码运行时会抛出NullPointerException?

Why does the following code throws NullPointerException when run?

import java.util.*;

class WhyNullPointerException {
    public static void main( String [] args ){
       // Create a map
        Map<String,Integer> m = new HashMap<String,Integer>();
        // Get the previous value, obviously null.
        Integer a = m.get( "oscar" );
        // If a is null put 1, else increase a
        int p = a == null ? 
            m.put( "oscar", 1) : 
            m.put( "oscar", a++ ); // Stacktrace reports Npe in this line
    }
}

推荐答案

因为 m.put 返回 null(表示没有前一个"值)当您尝试将其分配给 int 时.将 int p 替换为 Integer p 就可以了.

Because m.put returns null (which indicates that there's no "previous" value) while you're trying to assign it to int. Replace int p by Integer p and it will work.

这在 JLS 5.1.8 中有规定:

在运行时，拆箱转换过程如下:

5.1.8 Unboxing Conversion

At run time, unboxing conversion proceeds as follows:

• 如果 r 是 null，拆箱转换会抛出一个 NullPointerException

• If r is null, unboxing conversion throws a NullPointerException

---

与问题无关，只是考虑到 DRY 的附带建议，请考虑这么写:

---

Unrelated to the problem, just a side suggestion with DRY in mind, consider writing it so:

    Integer p = m.put("oscar", a == null ? 1 : a++);

它的可读性更强:)

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